Stents are commonly used to provide mechanical reinforcement for maintaining the patency of body passageways and cavities. Exemplary body passageways include blood vessels, the urethra, the bile duct, the esophagus, and the ureters.
A common use for stents is in the treatment of coronary artery disease. Coronary artery disease involves the narrowing or constricting of a coronary artery such that blood flow through the artery is diminished. Such a situation is commonly treated by balloon angioplasty procedures in which the afflicted artery is dilated/expanded through use of a balloon catheter. Without artificial reinforcement, the balloon expanded vessel has a tendency to constrict back to its previous obstructed internal diameter shortly after an angioplasty procedure. By implanting a stent along the expanded portion of the vessel, the vessel is provided with sufficient radial reinforcement to prevent the vessel from constricting.
Besides preventing vessel constriction, stents provide another important function when used in association with balloon angioplasty procedures. During a typical balloon angioplasty procedure, it is common for the afflicted artery to split or tear as it is expanded. The tearing of the vessel produces flaps of tissue that may project into the lumen of the vessel thereby interfering with blood flow. To prevent the aforementioned problem, a stent is implanted at the obstruction/constriction location. The stent compresses the flaps against the vessel to prevent interference with blood flow and to prevent the flaps from tearing from the vessel and entering the blood stream.
A variety of stent designs are known in the prior art. One category of stent design relates to self-expanding stents. Two types of self-expanding stents are prevalent in the medical field. The first type of self-expanding stent is fabricated of highly elastic material and exhibits elastic spring back characteristics. A common elastic self-expanding stent configuration comprises a helical stainless steel spring. The second type of self-expanding stent relates to temperature sensitive stents. Such temperature sensitive stents expand or contract in a radial direction depending upon their temperature.
An elastic self-expanding stent is typically manufactured in an enlarged orientation such that the stent has an enlarged diameter. The elastic self-expanding stent is then compressed to a compressed orientation in which the stent has a diameter sufficiently small to be inserted in a vessel. Once the stent is compressed to the compressed orientation, the stent is inserted within or on a catheter body that exerts sufficient radial stress upon the stent to prevent the stent from expanding. After the stent has been directed via the catheter to a desired location within a vessel, the elastic self-expanding stent is disengaged from the catheter. Upon disengagement from the catheter the radial compressive force provided by the catheter is removed and the stent automatically expands, via its elastic construction, from the compressed orientation to the expanded orientation. In the expanded orientation, the stent radially engages the vessel such that the vessel is reinforced to maintain the patency of the vessel. Additionally, contact between the vessel and the stent prevents the stent from migrating within the vessel.
A problem with elastic self-expanding stents is that they typically experience substantial length variations when transitioning between the compressed and expanded orientations. The significant length variation associated with such elastic self-expanding stents makes it difficult to precisely place the stents at a particular location within a body.
A temperature sensitive self-expanding stent is typically manufactured of a two-way shape-memory alloy such as Nitinol. Such a stent is designed to have an expanded orientation when the stent has a temperature above a shape transition temperature which is typically above room temperature and may be equal to body temperature. The temperature sensitive stent is radially compressed to a constricted orientation for implantation in a body vessel via a catheter. Once the stent is positioned at a desired location in the vessel, heat absorbed from the body causes the stent to expand to the expanded configuration. In the expanded configuration, the stent engages the vessel such that the vessel is radially reinforced.
Another category of stent relates to non-self-expanding stents. An exemplary non-self-expanding stent is disclosed in U.S. Pat. No. 4,733,665. Such stents typically are constructed of a deformable material. The deformable material allows the stents to maintain their shape when expanded. The stents are typically pre-equipped with a manufacturing installed catheter balloon and are typically manufactured with a small diameter sized to permit insertion into a vessel.
In use, a physician directs a non-self-expanding stent, via a catheter, to a desired location within a vessel. Once the stent reaches the desired location, the physician inflates the balloon in a controlled manner such that the stent is caused to expand. As the stent is expanded, the deformable material used to construct the stent deforms beyond its elastic limit. Consequently, the expanded stent has a tendency to maintain its expanded shape. Once expanded, the stent reinforces the vessel by providing radial reinforcement for maintaining the patency of the vessel.
A problem with a conventional non-self-expanding stent is that stress generated by a vessel in which the stent is implanted can cause the stent to be compressed radially inward thereby reducing the lumen size of the vessel. One reason for this problem is that when the non-self-expanding stent is expanded/deformed from a small diameter to an enlarged diameter, the individual members forming the stent have a tendency to twist relative to one another. The relative twisting of the members affects the structural integrity of the stent thereby reducing the stent's ability to resist vessel pressure.